An electrostatic chuck holds and supports a substrate during a manufacturing process and also removes heat from the substrate without mechanically clamping the substrate. During use of an electrostatic chuck, the back side of a substrate, such as a semiconductor wafer, is held to the face of the electrostatic chuck by an electrostatic force. The substrate is separated from one or more electrodes in the face of the electrostatic chuck by a surface layer of material that covers the electrode. In a Coulombic chuck, the surface layer is electrically insulating, while in a Johnsen-Rahbek electrostatic chuck, the surface layer is weakly conducting. The surface layer of the electrostatic chuck may be flat or may have one or more protrusions, projections or other surface features that further separate the back side of the substrate from the covered electrode. Heat delivered to the substrate during processing can be transferred away from the substrate and to the electrostatic chuck by contact heat conduction with the protrusions and/or by gas heat conduction with a cooling gas. Contact heat conduction is generally more efficient than gas heat conduction in removing heat from the substrate. However, controlling the amount of contact between the substrate and the protrusions can be difficult.
In microelectronics production, as semiconductor and memory device geometries become progressively smaller and the sizes of wafers, flat screen displays, reticles and other processed substrates become progressively larger, the allowable particulate contamination process specifications become more restrictive. The effect of particles on electrostatic chucks is of particular concern because the wafers physically contact or mount to the chuck clamping surface. If the mounting surface of the electrostatic chuck allows any particulate to become entrapped between the mounting surface and the substrate, the substrate may be deformed by the entrapped particle. For example, if the back side of a wafer is clamped electrostatically against a flat reference surface, the entrapped particle could cause a deformation of the front side of the wafer, which will therefore not lie in a flat plane. According to U.S. Pat. No. 6,835,415, studies have shown that a 10-micron particle on a flat electrostatic chuck can displace the surface of a reticle (i.e., a test wafer) for a radial distance of one inch or more. The actual height and diameter of the particle-induced displacement is dependent on numerous parameters such as the particle size, the particle hardness, the clamping force and the reticle thickness.
During substrate processing it is important to be able to control the temperature of the substrate, limit the maximum temperature rise of the substrate, maintain temperature uniformity over the substrate surface, or any combination of these. If there are excessive temperature variations across the substrate surface due to poor and/or non-uniform heat transfer, the substrate can become distorted and process chemistry can be affected. The greater the area of direct contact with the electrostatic chuck, the greater the heat transferred by contact heat conduction. The size of the area of direct contact is a function of the roughness, flatness and hardness of the contact surfaces of the substrate and electrostatic chuck, as well as of the applied pressure between the contact surfaces. Since the characteristics of the contact surface vary from substrate to substrate, and since the characteristics of the contact surface can change over time, accurately controlling contact heat conductance between the electrostatic chuck and substrate is difficult.
Controlling the temperature of a substrate and the number of particles on its back side is important for reducing or eliminating damage to microelectronic devices, reticle masks and other such structures, and for reducing or minimizing manufacturing yield loss. The abrasive properties of the electrostatic chuck protrusions, the high contact area of roughened protrusions, and the effect of lapping and polishing operations during manufacture of electrostatic chucks may all contribute adder particles to the back side of substrates during use with an electrostatic chuck.